A motor control device in an electric power steering determines a target value of current that should be passed through a motor on the basis of a steering torque and a vehicle speed. In determining the target value, for example, Japanese Patent Application Publication No. 8-67262 and Japanese Patent Application Publication No. 10-109655 describe that a steering speed or its corresponding motor angular velocity is considered in addition to a steering torque and a vehicle speed. When a brushless motor is controlled, an angle detection sensor, such as a resolver, for detecting a motor rotation position is usually used, so an output signal of the angle detection sensor is subjected to temporal differentiation to make it possible to obtain a motor angular velocity. However, in motor control in which no angle sensor is used, like control over a brushed motor, a motor current and a voltage between motor terminals are detected, and a motor angular velocity ω is calculated on the basis of the following mathematical expression.ω=(V−I×R)/k  (1)Here, V is a voltage between the motor terminals, I is a motor current, R is a motor resistance (resistance between the motor terminals), and k is a counter electromotive force constant.
Incidentally, in a brushed motor, the contact resistance of a brush varies depending on the motor current, so the motor resistance depends on the motor current. Therefore, in a motor control device that drives a brushed motor, the motor resistance R is obtained on the basis of the characteristic that expresses the correlation between a motor current and a motor resistance (hereinafter, referred to as “current-resistance characteristic of the motor”), and the obtained motor resistance R is used when a motor angular velocity is calculated using the above mathematical expression (1). By so doing, the motor angular velocity ω may be further accurately obtained.
However, the motor resistance R varies depending on a variation in temperature and unevenness in manufacturing. Therefore, there may be a difference between the value of motor resistance R that is used to calculate the motor angular velocity ω using the above mathematical expression (1) and the value of actual motor resistance. Thus, even when the motor resistance R that is obtained on the basis of the current-resistance characteristic of the motor is used to calculate the motor angular velocity ω, the motor angular velocity cannot always be calculated accurately because of the above variation in temperature and unevenness in manufacturing. As a result, for example, in an electric power steering that uses the above described motor control device, the accuracy of control that uses a motor angular velocity decreases, and there is a concern that steering assist cannot be performed appropriately.